Various control theories, for example, the skyhook theory, are known to be applied to a damping force control of a vehicle. In application of those theories, a shock absorber has characteristics that a vibration of sprung mass and a vibration of unsprung mass can be restrained if a high damping coefficient is selected, however, the shock absorber cannot restrain a high frequency component which is equal to or greater than a resonance frequency of the unsprung mass. To the contrary, if a low damping coefficient is selected, a vibration of sprung mass and a vibration of unsprung mass cannot be restrained. Particularly, in a case where the skyhook theory is applied, because restraining the vibration of the sprung mass is prioritized, when a vehicle travels on an unleveled or unpaved road surface, an input of the high frequency component which represents an unevenness of a road surface is increased.
For example, JPH05-294122A discloses an apparatus which modifies an actual damping coefficient to increase when a low frequency component of an oscillation input which is processed by a low-pass filter is greater, thus to enhance effects of a control based on the skyhook theory. However, the apparatus disclosed in JPH05-294122A is not constructed considering the high frequency component and the human sensitivity. According to JPH06-143965A, how the human feels and how to control the damping force to enhance the feeling of the occupants is considered in a case where an input in the vicinity of a resonance point of the sprung mass (floating feeling) and an input in the vicinity of a resonance point of the unsprung mass (feeling roughness) are simultaneously inputted. JPH06-143965A further proposes, the greater the high frequency component is in the oscillation of a vehicle body, to modify the damping force to the lower level in order to make the ride quality consistent with the operational stability at higher level even when the vehicle drives on the unleveled or unpaved road surface considering the human sensitivity. Regarding the view point of the human sensitivity, JP2911368B discloses the equation of motion of a vehicle which follows a preceding vehicle with a comfortable fore-aft movement which is consistent with the occupant's physiological senses. Further, the Weber Fechner law begins to be considered in various fields.
Notwithstanding, according to the construction disclosed in JPH05-294122A, although the vibration of sprung mass is actively restrained by increasing a gain in a case where the low frequency component is greater by applying the low-pass filter, there is a possibility that the ride quality is deteriorated because of feeling roughness when the high frequency component is inputted. Further, according to the construction disclosed in JPH06-143965A, although the damping force is modified to the lower level in response to the high frequency component even if the large number of low frequency components are included in light of the human sensitivity, the vibration of sprung mass becomes difficult to be restrained by the reduced degree of the damping force. Thus, the disclosure of JPH06-143965A is hardly the most efficient method for modifying the damping force when the human sensitivity is taken into consideration.
On the other hand, irrespective of the frequency bands such as the high frequency and the low frequency, focusing on the human's sensitivity, the sensitivity of the human to the effects obtained by controlling the damping force, for example, the sense that the oscillation is reduced, can be quantified using the Weber Fechner law. Accordingly, for example, by setting a value obtained by applying the Weber Fechner law as a modification gain, and by applying the modification gain to a known skyhook control, or the like, an optimum modification to the damping force control taking the human's sensitivity into consideration can be performed.
A need thus exists for a damping force control apparatus which is not susceptible to the drawback mentioned above